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annotate Doc/library/sets.rst @ 54294:dba767bc3155 legacy-trunk

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remove mention of old ctypes version
author Benjamin Peterson <benjamin@python.org>
date Sat, 23 May 2009 20:59:09 +0000
parents 31cf38bdaac0
children 3e15a07a32fa
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2 :mod:`sets` --- Unordered collections of unique elements
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3 ========================================================
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5 .. module:: sets
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6 :synopsis: Implementation of sets of unique elements.
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7 :deprecated:
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8 .. moduleauthor:: Greg V. Wilson <gvwilson@nevex.com>
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9 .. moduleauthor:: Alex Martelli <aleax@aleax.it>
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10 .. moduleauthor:: Guido van Rossum <guido@python.org>
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11 .. sectionauthor:: Raymond D. Hettinger <python@rcn.com>
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14 .. versionadded:: 2.3
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16 .. deprecated:: 2.6
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17 The built-in ``set``/``frozenset`` types replace this module.
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18
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19 The :mod:`sets` module provides classes for constructing and manipulating
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20 unordered collections of unique elements. Common uses include membership
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21 testing, removing duplicates from a sequence, and computing standard math
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22 operations on sets such as intersection, union, difference, and symmetric
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23 difference.
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24
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25 Like other collections, sets support ``x in set``, ``len(set)``, and ``for x in
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26 set``. Being an unordered collection, sets do not record element position or
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27 order of insertion. Accordingly, sets do not support indexing, slicing, or
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28 other sequence-like behavior.
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29
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30 Most set applications use the :class:`Set` class which provides every set method
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31 except for :meth:`__hash__`. For advanced applications requiring a hash method,
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32 the :class:`ImmutableSet` class adds a :meth:`__hash__` method but omits methods
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33 which alter the contents of the set. Both :class:`Set` and :class:`ImmutableSet`
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34 derive from :class:`BaseSet`, an abstract class useful for determining whether
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35 something is a set: ``isinstance(obj, BaseSet)``.
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36
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37 The set classes are implemented using dictionaries. Accordingly, the
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38 requirements for set elements are the same as those for dictionary keys; namely,
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39 that the element defines both :meth:`__eq__` and :meth:`__hash__`. As a result,
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40 sets cannot contain mutable elements such as lists or dictionaries. However,
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41 they can contain immutable collections such as tuples or instances of
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42 :class:`ImmutableSet`. For convenience in implementing sets of sets, inner sets
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43 are automatically converted to immutable form, for example,
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44 ``Set([Set(['dog'])])`` is transformed to ``Set([ImmutableSet(['dog'])])``.
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47 .. class:: Set([iterable])
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48
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49 Constructs a new empty :class:`Set` object. If the optional *iterable*
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50 parameter is supplied, updates the set with elements obtained from iteration.
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51 All of the elements in *iterable* should be immutable or be transformable to an
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52 immutable using the protocol described in section :ref:`immutable-transforms`.
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53
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55 .. class:: ImmutableSet([iterable])
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56
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57 Constructs a new empty :class:`ImmutableSet` object. If the optional *iterable*
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58 parameter is supplied, updates the set with elements obtained from iteration.
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59 All of the elements in *iterable* should be immutable or be transformable to an
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60 immutable using the protocol described in section :ref:`immutable-transforms`.
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61
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62 Because :class:`ImmutableSet` objects provide a :meth:`__hash__` method, they
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63 can be used as set elements or as dictionary keys. :class:`ImmutableSet`
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64 objects do not have methods for adding or removing elements, so all of the
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65 elements must be known when the constructor is called.
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68 .. _set-objects:
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69
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70 Set Objects
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71 -----------
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72
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73 Instances of :class:`Set` and :class:`ImmutableSet` both provide the following
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74 operations:
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75
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76 +-------------------------------+------------+---------------------------------+
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77 | Operation | Equivalent | Result |
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78 +===============================+============+=================================+
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79 | ``len(s)`` | | cardinality of set *s* |
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80 +-------------------------------+------------+---------------------------------+
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81 | ``x in s`` | | test *x* for membership in *s* |
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82 +-------------------------------+------------+---------------------------------+
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83 | ``x not in s`` | | test *x* for non-membership in |
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84 | | | *s* |
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85 +-------------------------------+------------+---------------------------------+
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86 | ``s.issubset(t)`` | ``s <= t`` | test whether every element in |
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87 | | | *s* is in *t* |
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88 +-------------------------------+------------+---------------------------------+
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89 | ``s.issuperset(t)`` | ``s >= t`` | test whether every element in |
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90 | | | *t* is in *s* |
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91 +-------------------------------+------------+---------------------------------+
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92 | ``s.union(t)`` | ``s | t`` | new set with elements from both |
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93 | | | *s* and *t* |
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94 +-------------------------------+------------+---------------------------------+
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95 | ``s.intersection(t)`` | ``s & t`` | new set with elements common to |
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96 | | | *s* and *t* |
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97 +-------------------------------+------------+---------------------------------+
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98 | ``s.difference(t)`` | ``s - t`` | new set with elements in *s* |
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99 | | | but not in *t* |
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100 +-------------------------------+------------+---------------------------------+
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101 | ``s.symmetric_difference(t)`` | ``s ^ t`` | new set with elements in either |
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102 | | | *s* or *t* but not both |
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103 +-------------------------------+------------+---------------------------------+
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104 | ``s.copy()`` | | new set with a shallow copy of |
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105 | | | *s* |
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106 +-------------------------------+------------+---------------------------------+
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107
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108 Note, the non-operator versions of :meth:`union`, :meth:`intersection`,
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109 :meth:`difference`, and :meth:`symmetric_difference` will accept any iterable as
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110 an argument. In contrast, their operator based counterparts require their
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111 arguments to be sets. This precludes error-prone constructions like
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112 ``Set('abc') & 'cbs'`` in favor of the more readable
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113 ``Set('abc').intersection('cbs')``.
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114
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115 .. versionchanged:: 2.3.1
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116 Formerly all arguments were required to be sets.
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117
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118 In addition, both :class:`Set` and :class:`ImmutableSet` support set to set
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119 comparisons. Two sets are equal if and only if every element of each set is
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120 contained in the other (each is a subset of the other). A set is less than
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121 another set if and only if the first set is a proper subset of the second set
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122 (is a subset, but is not equal). A set is greater than another set if and only
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123 if the first set is a proper superset of the second set (is a superset, but is
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124 not equal).
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125
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126 The subset and equality comparisons do not generalize to a complete ordering
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127 function. For example, any two disjoint sets are not equal and are not subsets
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128 of each other, so *all* of the following return ``False``: ``a<b``, ``a==b``,
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129 or ``a>b``. Accordingly, sets do not implement the :meth:`__cmp__` method.
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130
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131 Since sets only define partial ordering (subset relationships), the output of
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132 the :meth:`list.sort` method is undefined for lists of sets.
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133
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134 The following table lists operations available in :class:`ImmutableSet` but not
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135 found in :class:`Set`:
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136
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137 +-------------+------------------------------+
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138 | Operation | Result |
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139 +=============+==============================+
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140 | ``hash(s)`` | returns a hash value for *s* |
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141 +-------------+------------------------------+
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142
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143 The following table lists operations available in :class:`Set` but not found in
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144 :class:`ImmutableSet`:
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145
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146 +--------------------------------------+-------------+---------------------------------+
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147 | Operation | Equivalent | Result |
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148 +======================================+=============+=================================+
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149 | ``s.update(t)`` | *s* \|= *t* | return set *s* with elements |
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150 | | | added from *t* |
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151 +--------------------------------------+-------------+---------------------------------+
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152 | ``s.intersection_update(t)`` | *s* &= *t* | return set *s* keeping only |
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153 | | | elements also found in *t* |
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154 +--------------------------------------+-------------+---------------------------------+
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155 | ``s.difference_update(t)`` | *s* -= *t* | return set *s* after removing |
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156 | | | elements found in *t* |
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157 +--------------------------------------+-------------+---------------------------------+
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158 | ``s.symmetric_difference_update(t)`` | *s* ^= *t* | return set *s* with elements |
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159 | | | from *s* or *t* but not both |
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160 +--------------------------------------+-------------+---------------------------------+
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161 | ``s.add(x)`` | | add element *x* to set *s* |
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162 +--------------------------------------+-------------+---------------------------------+
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163 | ``s.remove(x)`` | | remove *x* from set *s*; raises |
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164 | | | :exc:`KeyError` if not present |
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165 +--------------------------------------+-------------+---------------------------------+
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166 | ``s.discard(x)`` | | removes *x* from set *s* if |
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167 | | | present |
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168 +--------------------------------------+-------------+---------------------------------+
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169 | ``s.pop()`` | | remove and return an arbitrary |
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170 | | | element from *s*; raises |
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171 | | | :exc:`KeyError` if empty |
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172 +--------------------------------------+-------------+---------------------------------+
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173 | ``s.clear()`` | | remove all elements from set |
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174 | | | *s* |
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175 +--------------------------------------+-------------+---------------------------------+
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176
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177 Note, the non-operator versions of :meth:`update`, :meth:`intersection_update`,
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178 :meth:`difference_update`, and :meth:`symmetric_difference_update` will accept
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179 any iterable as an argument.
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180
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181 .. versionchanged:: 2.3.1
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182 Formerly all arguments were required to be sets.
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183
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184 Also note, the module also includes a :meth:`union_update` method which is an
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185 alias for :meth:`update`. The method is included for backwards compatibility.
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186 Programmers should prefer the :meth:`update` method because it is supported by
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187 the builtin :class:`set()` and :class:`frozenset()` types.
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188
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189
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190 .. _set-example:
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191
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192 Example
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193 -------
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194
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195 >>> from sets import Set
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196 >>> engineers = Set(['John', 'Jane', 'Jack', 'Janice'])
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197 >>> programmers = Set(['Jack', 'Sam', 'Susan', 'Janice'])
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198 >>> managers = Set(['Jane', 'Jack', 'Susan', 'Zack'])
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199 >>> employees = engineers | programmers | managers # union
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200 >>> engineering_management = engineers & managers # intersection
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201 >>> fulltime_management = managers - engineers - programmers # difference
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202 >>> engineers.add('Marvin') # add element
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203 >>> print engineers # doctest: +SKIP
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204 Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
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205 >>> employees.issuperset(engineers) # superset test
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206 False
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207 >>> employees.update(engineers) # update from another set
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208 >>> employees.issuperset(engineers)
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209 True
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210 >>> for group in [engineers, programmers, managers, employees]: # doctest: +SKIP
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211 ... group.discard('Susan') # unconditionally remove element
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212 ... print group
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213 ...
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214 Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
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215 Set(['Janice', 'Jack', 'Sam'])
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216 Set(['Jane', 'Zack', 'Jack'])
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217 Set(['Jack', 'Sam', 'Jane', 'Marvin', 'Janice', 'John', 'Zack'])
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218
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219
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220 .. _immutable-transforms:
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221
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222 Protocol for automatic conversion to immutable
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223 ----------------------------------------------
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224
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225 Sets can only contain immutable elements. For convenience, mutable :class:`Set`
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226 objects are automatically copied to an :class:`ImmutableSet` before being added
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227 as a set element.
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228
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229 The mechanism is to always add a :term:`hashable` element, or if it is not
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230 hashable, the element is checked to see if it has an :meth:`__as_immutable__`
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231 method which returns an immutable equivalent.
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232
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233 Since :class:`Set` objects have a :meth:`__as_immutable__` method returning an
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234 instance of :class:`ImmutableSet`, it is possible to construct sets of sets.
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235
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236 A similar mechanism is needed by the :meth:`__contains__` and :meth:`remove`
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237 methods which need to hash an element to check for membership in a set. Those
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238 methods check an element for hashability and, if not, check for a
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239 :meth:`__as_temporarily_immutable__` method which returns the element wrapped by
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240 a class that provides temporary methods for :meth:`__hash__`, :meth:`__eq__`,
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241 and :meth:`__ne__`.
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242
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243 The alternate mechanism spares the need to build a separate copy of the original
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244 mutable object.
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245
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246 :class:`Set` objects implement the :meth:`__as_temporarily_immutable__` method
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247 which returns the :class:`Set` object wrapped by a new class
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248 :class:`_TemporarilyImmutableSet`.
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249
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250 The two mechanisms for adding hashability are normally invisible to the user;
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251 however, a conflict can arise in a multi-threaded environment where one thread
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252 is updating a set while another has temporarily wrapped it in
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253 :class:`_TemporarilyImmutableSet`. In other words, sets of mutable sets are not
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254 thread-safe.
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255
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256
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257 .. _comparison-to-builtin-set:
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258
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259 Comparison to the built-in :class:`set` types
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260 ---------------------------------------------
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261
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262 The built-in :class:`set` and :class:`frozenset` types were designed based on
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263 lessons learned from the :mod:`sets` module. The key differences are:
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264
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265 * :class:`Set` and :class:`ImmutableSet` were renamed to :class:`set` and
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266 :class:`frozenset`.
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267
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268 * There is no equivalent to :class:`BaseSet`. Instead, use ``isinstance(x,
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269 (set, frozenset))``.
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270
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271 * The hash algorithm for the built-ins performs significantly better (fewer
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272 collisions) for most datasets.
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273
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274 * The built-in versions have more space efficient pickles.
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275
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276 * The built-in versions do not have a :meth:`union_update` method. Instead, use
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277 the :meth:`update` method which is equivalent.
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278
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279 * The built-in versions do not have a ``_repr(sorted=True)`` method.
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280 Instead, use the built-in :func:`repr` and :func:`sorted` functions:
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281 ``repr(sorted(s))``.
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282
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283 * The built-in version does not have a protocol for automatic conversion to
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284 immutable. Many found this feature to be confusing and no one in the community
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285 reported having found real uses for it.
c62f6669ae22 Move the 2.6 reST doc tree in place.
Georg Brandl <georg@python.org>
parents:
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